"""i18n: locale resolution + a route to switch UI language.

Order: explicit session choice -> authenticated user.locale -> Accept-Language -> default.
"""
from flask import Blueprint, session, redirect, request, current_app, url_for
from flask_login import current_user
from app.utils import safe_referrer

i18n_bp = Blueprint("i18n", __name__)


def select_locale():
    supported = current_app.config["SUPPORTED_LOCALES"]
    # 1. explicit session override
    lang = session.get("lang")
    if lang in supported:
        return lang
    # 2. authenticated user's stored preference
    if current_user.is_authenticated and current_user.locale in supported:
        return current_user.locale
    # 3. browser preference
    best = request.accept_languages.best_match(supported)
    if best:
        return best
    # 4. default
    return current_app.config["DEFAULT_LOCALE"]


@i18n_bp.route("/lang/<code>")
def set_lang(code):
    if code in current_app.config["SUPPORTED_LOCALES"]:
        session["lang"] = code
        # persist to profile when logged in
        if current_user.is_authenticated:
            from app.extensions import db
            current_user.locale = code
            db.session.commit()
    return redirect(safe_referrer(url_for("main.index")))